The present invention concerns a system for the delivery of gas stored in a vessel in liquefied form, said vessel including in its lower part a liquefied phase of said gas and in the upper part a gaseous phase of said gas, this vessel including a means for connecting the vessel to a means for utilization as well as a means for heating the lower part of said vessel.
The semiconductor industry is today confronted with growing needs for so-called specialty gases for the various steps necessary for the fabrication of integrated circuits. Some of these specialty gases, such as HCl, Cl2, HBr, N2O, NH3, WF6, BCl3, and 3MS, to cite only some of them, liquefy at ambient temperature, and because of this fact pose difficulties in their distribution. These difficulties are directly related to their pressure and/or their flow rate during utilization.
A liquefied gas is composed of two phases, liquid and gaseous, in equilibrium with each other. This equilibrium implies that at a given temperature a liquefied gas has a well-determined pressure and that this pressure varies as a function of the temperature according to a relationship that is specific to each gas. Thus, FIG. 1 shows an equilibrium curve for the liquid and vapor phases of trimethylsilane (referred to as 3MS), which indicates the pressure of the gas in equilibrium above the liquid phase as a function of temperature. It is found that the pressure increases as the temperature increases, and vice versa.
When the gaseous phase is withdrawn from a tank of liquefied gas, part of the liquid must be converted into gas to regenerate the gas in proportion to the amount used in order to maintain the equilibrium. The liquid thus begins to boil using the available energy (typically the energy of the external medium surrounding the tank). As the rate of withdrawal is increased, this energy requirement will increase, and the liquid will boil violently, thus creating a substantial risk of entrainment of impurity-loaded droplets in the gaseous phase. These droplets not only contaminate the gas but also accelerate corrosion processes and cause instabilities with regard to regulation of the flow rate and pressure measurements. If the available energy is insufficient to gasify the liquid and thus regenerate the vapor phase, the temperature—and thus the pressure—will drop since the equilibrium must be maintained.
An external contribution of energy through heating makes it possible to limit the cooling and pressure drop observed. Several solutions are thereby conceivable.
One solution illustrated by FIG. 1 comprises heating the foot or bottom of the tank while controlling the heating using the pressure in the tank. Heating is allowed when the pressure is below the pressure that corresponds to ambient temperature, and heating is stopped when the liquid reaches or is at ambient temperature. By keeping the gas at a temperature slightly below ambient temperature, it is possible to avoid having to lay out the distribution network under the restriction that there be no cold point along it. Such a system is described in U.S. Pat. Nos. 5,761,911, 6,076,359, and 6,199,384.
In general the heating techniques used up to now to increase the flow rates of liquefied gases comprise heating the body of the tank using a resistive heating element of the heating belt or heating ribbon type, or even hot air. This type of heating has the drawback that energy transfer is substantially limited by the thermal conduction from the heating element to the tank, which results in a limitation of the usable flow rate despite a substantial energy input. In other words, such installations have a low energy efficiency.
More generally there is the problem of increasing the flow rate for a gas coming from a tank where the gas is stored in liquid form. Another technical problem arises when it is desired to increase the pressure of the gas delivered by the tank above its equilibrium pressure with respect to the liquid in the tank at ambient temperature. In both of these cases one solution that can be employed is that described in the patents referenced above, by increasing the power transferred by the heating system. In this case, it is quickly established that the heating system can reach a temperature above 100° C., the heating energy being transmitted by conduction to the tank and/or to the liquid, producing an increase in the temperature of the tank, at least locally, such that impurities absorbed on the tank walls, such as CO, CO2, etc., undergo desorption, which results in the delivery of gas containing impurities such as CO, CO2, etc., which is unacceptable for the user, particularly in the field of semiconductor fabrication (but also in other technical fields).
Thus, we are today confronted with the problem of increasing the flow rate and/or the pressure of the gas delivered by a reservoir (tank, etc.) without producing additional impurities, which would run counter to the intended purpose (since on the contrary the vaporization of the gas already makes it possible to eliminate the impurities present in the liquid that are not readily vaporizable).